Many electrical devices are incorporating touchscreen type displays. A touchscreen is a display that detects the presence, location, and pressure of a touch within the display area, generally by a finger, hand, stylus, or other pointing device. The touchscreen enables a user to interact with the display panel directly without requiring any intermediate device, rather than indirectly with a mouse or touchpad. Touchscreens can be implemented in computers or as terminals to access networks. Touchscreens are commonly found in point-of-sale systems, automated teller machines (ATMs), mobile phones, personal digital assistants (PDAs), portable game consoles, satellite navigation devices, and information appliances.
There are many types of touchscreen technologies. A resistive touchscreen panel is composed of several layers including two thin metallic electrically conductive and resistive layers separated by a thin space. When an object touches the touchscreen panel, the layers are connected at certain point. In response to the object contact, the panel electrically acts similar to two voltage dividers with connected outputs. This causes a change in an electrical current that is registered as a touch event and sent to the controller for processing.
A capacitive touchscreen panel is coated, partially coated, or patterned with a material that conducts a continuous electrical current across a sensor. The sensor exhibits a precisely controlled field of stored electrons in both the horizontal and vertical axes to achieve capacitance. The human body is conductive and therefore influences electric fields stored in a capacitance. When a reference capacitance of the sensor is altered by another capacitance field, such as a finger, electronic circuits located at each corner of the panel measure the resultant distortion in the reference capacitance. The measured information related to the touch event is sent to the controller for mathematical processing. Capacitive sensors can either be touched with a bare finger or with a conductive device being held by a bare hand. Capacitive sensors also work based on proximity, and do not have to be directly touched to be triggered. In most cases, direct contact to a conductive metal surface does not occur and the conductive sensor is separated from the user's body by an insulating glass or plastic layer. Devices with capacitive buttons intended to be touched by a finger can often be triggered by quickly waving the palm of the hand close to the surface without touching.
FIG. 1 illustrates an exemplary conventional capacitive touch sensor used in a capacitive touchscreen panel. Such sensors are typically formed using transparent conductors, such as ITO (Indium Tin Oxide) conductors, formed in layers. In the exemplary configuration of FIG. 1, bottom conductors form drive electrodes X0, X1, X2, X3, also referred to as drive lines, and top conductors form sense electrodes Y0, Y1, Y2, Y3, also referred to as sense lines. Each cross-point of a drive line and a sense line forms a capacitor having a measured capacitance Cm. The objective is to determine an estimate of a touch position on the capacitive touch sensor. When a finger, or other object that is grounded, is positioned on or proximate a cross-point of the sensor, there is a change in the measured capacitance Cm at that cross-point. The measured capacitance Cm is the capacitance between the sense line and the drive line at the cross-point. When the touch event occurs at the cross-point, a portion of the field lines between the sense line and the drive line are diverted to between the sense line and the finger. As such the measured capacitance Cm decreases during a touch event.
An analog front end (AFE) circuit performs signal processing on an analog signal and typically performs an analog-to-digital conversion. Analog front end circuits can be used in a variety of applications, including measuring and converting a capacitance to a corresponding voltage. FIGS. 2A and 2B illustrate a simplified schematic block diagram of a conventional analog front end circuit used for measuring a capacitance of an external capacitor and converting the measured capacitance to a corresponding voltage. In an exemplary application, the external capacitance is the charge stored in the capacitor Cm of FIG. 1. FIG. 2A illustrates the circuit in a first phase, and FIG. 2B illustrates the circuit in a second phase. During phase 1, charge to be measured is collected on the capacitor Cm. During phase 2, the charge stored on the capacitor Cm is transferred to the capacitor Cf and a corresponding voltage Vout is output.
Referring to FIG. 2A, the circuit includes the capacitor Cm, an operational amplifier 2, a switch 4, a feedback capacitor Cf, and a switch 6. A voltage at the negative input of the amplifier 2, and therefore at a first terminal of the capacitor Cm, is a virtual ground, Vvg. During phase 1, the switch 4 is connected to the reference voltage Vref, and the switch 6 is closed. Closing the switch 6 enables the capacitor Cf to completely discharge to a known zero state. The charge across the capacitor Cm is (Vvg-Vref) times the capacitance Cm.
During phase 2, the switch 4 is connected to ground, and the switch 6 is opened, as shown in FIG. 2B. With the switch 4 connected to ground the voltage across the capacitor Cm is zero, and all the charge on the capacitor Cm is transferred to the capacitor Cf. The output voltage Vout is a signal with amplitude dependent on the charge stored on the capacitor Cm and transferred to the capacitor Cf. The output voltage Vout can be input to an analog-to-digital converter (ADC), such as in FIG. 4, to be converted to a corresponding digital output value. Since the capacitor Cf was completely discharged during phase 1, the charge stored on capacitor Cf is determined entirely by the amount of charge transferred from the capacitor Cm. If the capacitor Cf is not completely discharged to a zero state during phase 1, then the capacitor Cf will retain the memory of its previous state.
The output voltage Vout=(Vref*Cm/Cf)+vn, where Vref is a known internal reference value, vn is the undesired noise measured by the system, and Cf is a known value. As such, the capacitance Cm can be determined from the known values Vref and Cf and the measured value Vout. The capacitance Cm is a varying capacitance and represents the capacitance to be measured, such as the measured capacitance of a touch screen display. As a finger touches the touch screen display, the capacitance changes, which is the external capacitance change being measured.
A problem with the circuit of FIGS. 2A and 2B relates to wide-band noise sampling. The circuit does not include any noise filtering, so any noise introduced into the system at the transition from phase 1 to phase 2 is included within the charge transferred to the capacitor Cf. This noise is represented as the component “vn” in the output voltage Vout. So not only is the output voltage Vout a measure of the capacitance Cm, but also an instantaneous sampling of the noise. Further, the dynamic range of the ADC needs to be large enough to account for the increased magnitude of the output voltage Vout due to noise. The larger dynamic range results in an ADC that has a larger area and uses more power.
FIG. 3 illustrates exemplary response curves for the circuit of FIGS. 2A and 2B. The top curve shows a sampling clock corresponding to phase 1 and phase 2. When the sample clock is high, e.g. 1V, the circuit is in phase 1 (FIG. 1), and when the sample clock is low, e.g. 0V, the circuit is in phase 2 (FIG. 2). In an exemplary application, the input is sampled on the rising edge of the sampling clock. The moment that the switches 4 and 6 are changed from phase 2 to phase 1 the voltage Vout is sampled. As shown in the middle curve of FIG. 3, there is some noise on the input signal, but its average value is substantially constant. The sampled value is expected to be constant, such as 1V, but due to the noise the actual sampled output varies about the expected constant value depending on the instantaneous noise present at the sampling time. An example of this variation on the actual sampled output is shown in the bottom curve of FIG. 3. If the instantaneous noise is high, then the actual sampled output is greater than the expected constant value, such as the portions of the sampled output curve that are above 1V. If the instantaneous noise is low, then the actual sampled output is lower than the expected constant value, such as the portions of the sampled output curve that are below 1V.
In application, a threshold voltage for determining a change in capacitance, such as a touch event on a touch screen display, is increased to accommodate the variation in the sampled output. Increasing the threshold voltage reduces the sensitivity of the system. Using a threshold voltage that is too low to account for the noise variations results in false triggers.
Various alternative systems that measure a capacitance include considerations for the noise. FIG. 4 illustrates a simplified schematic block diagram of a conventional analog front end circuit using digital filtering. The circuit of FIG. 4 includes an analog-to-digital converter (ADC) connected to the output of the low-noise amplifier (LNA). Voltage input to the ADC is converted to a digital value, which is processed by digital processing circuitry that includes noise filtering. The ADC is also a sampling system which samples at a single instant in time. This results in similar varying sampled output values as described above in relation to FIG. 3.
FIG. 5 illustrates a simplified schematic block diagram of another conventional analog front end circuit. The circuit of FIG. 5 is the same as the circuit of FIG. 4 with the addition of a band-pass filter (BPF) to filter the signal prior to inputting to the ADC. The BPF attempts to filter the noise present in the voltage signal (middle curve of FIG. 3) prior to inputting to the ADC. Sampling is performed on the filtered signal output from the BPF. The problem with the circuit of FIG. 5 is that different applications are subjected to different noise spectrums. As such, the BPF cannot be fixed, instead the BPF must be tunable to the specific application. Also, the BPF should be able to be finely tuned to accommodate applications with a relatively narrow frequency response. For example, a touch screen display may have a frequency response between about 50-400 kHz. If the BPF has too large a bandwidth, such as 50 kHz, the filter bandwidth may be too wide to effectively filter noise for certain applications.